What is the Difference Between Heterosis and Inbreeding Depression

The main difference between heterosis and inbreeding depression is that heterosis consists of beneficial augmentations of phenotypic trait values in offspring of genetically distant parents. In contrast, inbreeding depression consists of detrimental reductions of phenotypic trait values in the offspring of genetically related parents. Therefore, heterosis occurs as a result of outbreeding enhancement, while inbreeding depression occurs as a result of inbreeding. Furthermore, heterosis is a consequence of increased offspring heterozygosity, while inbreeding depression is a consequence of increased homozygosity of the offspring. 

In brief, heterosis and inbreeding depression are two effects of parental distance, affecting offspring fitness. Generally, the effects of inbreeding and outbreeding in natural populations are the major goal in evolutionary biology.  

Key Areas Covered 

1. What is Heterosis
     – Definition, Process, Effect on Evolution
2. What is Inbreeding Depression
     – Definition, Process, Effect on Evolution
3. What are the Similarities Between Heterosis and Inbreeding Depression
     – Outline of Common Features
4. What is the Difference Between Heterosis and Inbreeding Depression
     – Comparison of Key Differences

Key Terms 

Biological Quality of a Phenotype, Heterosis, Hybrid Vigor, Inbreeding Depression

What is Heterosis 

Heterosis is the improved biological quality of a phenotypic trait in the progeny of diverse varieties of a species or crosses between species. Therefore, the progeny may exhibit greater biomass, speed of development, fertility, etc. than both parents. On that account, heterosis is also known as hybrid vigor or outbreeding enhancement. In general, outbreeding tends to increase the genetic diversity of the progeny. Consequently, it increases the biological fitness of the offspring.

Figure 1: Heterosis
A. Dominance, B. Overdominance

Besides, various models are there to describe heterosis, including dominance, codominance, overdominance, pseudo-overdominance. Of these, codominance is the more popular model. In which, one parent contains gene copies, which are missing in the opposite parent and thus, the hybrid would contain more genes than either parent, resulting in hybrid vigor. As an example, hybrids between radish (Raphanus sativus) and cabbage (Brassica oleracea) exhibit extensive biomass heterosis. Meanwhile, in dominance, fewer genes undergo expression in the homozygous individual. Therefore, gene expression in the offspring is equal to the expression of the fittest parent.  On the other hand, in overdominance, certain genes in the heterozygous offspring undergo overexpression. 

What is Inbreeding Depression 

Inbreeding depression is the decreased biological quality of a phenotypic trait in the progeny of related varieties of a species. Furthermore, in small populations or inbred, it tends to lose genetic diversity. Hence, inbreeding depression is the loss of fitness due to reduced genetic diversity. Usually, the biological fitness of an organism refers to an organism’s ability to survive and perpetuate its genetic material. However, higher genetic variation or gene pool within a breeding population makes it less suffer from inbreeding depression.

Figure 2: Inbreeding Depression

Generally, inbreeding results in the accumulation of recessive traits in the offspring as the pair-mates are more similar. Consequently, the offspring becomes homozygous to the deleterious genes, making it more unfit.  

Similarities Between Heterosis and Inbreeding Depression 

• Heterosis and inbreeding depression are two effects of parental distance, which affects the fitness of the offspring. 
• They are the consequences of inbreeding and outbreeding in natural populations. 
• Both are major goals of evolutionary biology, determining the quality of a particular phenotype. 

Difference Between Heterosis and Inbreeding Depression 

Definition 

Heterosis refers to the increased function of any biological quality in a hybrid offspring, while inbreeding depression refers to the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals.  

Genetic Variation in Parents 

Heterosis occurs in the offspring of genetically distant parents, while inbreeding depression occurs in the offspring of genetically related parents. 

Homozygosity/Heterozygosity 

Heterosis is a consequence of increased offspring heterozygosity, while inbreeding depression is a consequence of increased homozygosity of the offspring. 

The appearance of Genetic Variation in Parents 

In heterosis, genetic variation can appear in F1 individuals of genetically uniform populations, but in inbreeding depression, genetic variation must be present within the population. 

Occurrence 

Heterosis occurs as a result of outbreeding enhancement, while inbreeding depression occurs as a result of inbreeding.  

Effect on the Phenotypic Trait 

Heterosis consists of beneficial augmentations of phenotypic trait values. In contrast, inbreeding depression consists of detrimental reductions. 

Effect of Genetic Drift of Small Populations 

Heterosis is higher in small populations, but inbreeding depression is lower in small populations. 

Likelihood of Outbreeding Depression and Its Consequences 

Outbreeding depression may lower the magnitude of heterosis while outbreeding depression is unlikely to affect the magnitude of inbreeding depression without isolation or local adaptation. 

Conclusion 

In brief, heterosis is the increased biological quality of a phenotypic trait. Generally, it occurs as a result of outbreeding enhancement in the offspring of genetically distant parents. Also, heterosis is higher in small populations. In contrast, inbreeding depression is the decreased biological quality of a phenotypical trait. Usually, it occurs as a result of inbreeding in the offspring of genetically related parents. However, inbreeding depression is lower in small populations. Therefore, the main difference between heterosis and inbreeding depression is the effect on the biological quality of the phenotypic trait.    

References:

1. “Heterosis versus Inbreeding Depression Effects: An Investigation of Personality, Digit Ratio (2D:4D), and Laterality Traits.” Mankind Quarterly, vol. 52, no. 2, 2011, p. 191 . ProQuest LLC. Available Here.
2. Birchler, James A et al. “Heterosis.” The Plant cell vol. 22,7 (2010): 2105-12. doi:10.1105/tpc.110.076133
3. “Inbreeding Depression.” Understanding Evolution, Available Here.

Image Courtesy:

1. “Heterosis” By User:Mysid (Public Domain) via Commons Wikimedia   
2. “Shetland pony inbred” By en:User:ImaginaryFriend (Public Domain) via Commons Wikimedia